1. Field of the Invention
The present invention relates to a power transmission device of an engine and, more particularly to a power transmission device of an engine having a mechanism for converting a rotational direction of an output of the engine by substantially 90 degrees.
2. Description of the Related Art
There are various types of engines according to uses. For example, an engine includes a drive shaft extending in a direction orthogonal to a crank shaft of the engine. Such an engine is adopted as an engine of a shaft drive type motor cycle, an engine of a straddle-type four wheel all terrain vehicle, or the like.
FIG. 5 is a partially sectioned side view showing an engine 101 of a straddle-type four wheel all terrain vehicle disclosed in Japanese Laid-Open Patent Publication No. Sho. 60-237232. Referring to FIG. 5, in the engine 101, a drive shaft 120 is placed below a crank shaft 110. The drive shaft 120 extends such that it makes substantially 90 degrees with respect to the crank shaft 110. In a crank chamber 139, a wall portion 134 is formed rear of the crank shaft 110 but there is no wall portion formed below the crank shaft 110. The reason why no wall portion is formed below the crank shaft 110 is that the drive shaft 120 is placed close to the crank shaft 110. The drive shaft 120 is placed close to the crank shaft 110 to make the engine 101 small.
As describe above, to realize that the engine 101 in which the drive shaft 120 is placed below the crank shaft 110 and the drive shaft 120 makes substantially 90 degrees with respect to the crank shaft 110 is made small, it is desirable to place the drive shaft 120 as close to the crank shaft 110 as possible.
The crank shaft 110 is a member at which a rotational driving force is generated in the engine 101. The drive shaft 120 is a member from which the rotational driving force is output to outside of the engine 101. These shafts are members to be firmly supported by a crank case. In order to firmly support the crank shaft 110 and the drive shaft 120, it is necessary to form a chamber accommodating the crank shaft 110 and a chamber accommodating the drive shaft 120 such that these chambers have sufficient rigidity.
In order to ensure the rigidity of the chamber accommodating the drive shaft 120 and the rigidity of the chamber accommodating the crank shaft 110 and firmly support the drive shaft 120 and the crank shaft 110, it is desirable to form a separating wall integrated with the crank case between the drive shaft 120 and the crank shaft 110.
When forming the separating wall, it is necessary to position the separating wall so as to prevent a crank pin and a crank web of the crank shaft 110, a connecting rod and the like, from coming into contact with the separating wall when they are rotating. Therefore, when an attempt is made to place the drive shaft 120 just below the crank pin and to position the separating wall having sufficient rigidity so as to prevent the rotating crank pin or the rotating connecting rod from coming into contact with separating wall, it is necessary to place the drive shaft 120 considerably below the crank shaft 110. Such placement makes the engine 101 large.
The present invention has been developed for obviating the above-described problem, and an object of the present invention is to provide a power transmission device of an engine which makes the engine small and enables a crank shaft and a drive shaft to be supported by a sufficiently rigid crank case.
To solve the above-described problem, there is provided a power transmission device of an engine comprising; a crank shaft; a drive shaft provided such that the drivel shaft makes substantially 90 degrees with respect to the crank shaft, a rotational driving force from the crank shaft being transmitted to the drive shaft; a crank case; a first bearing for rotatably supporting the crank shaft with respect to the crank case; and a second bearing for rotatably supporting the drive shaft, wherein a tubular portion is formed integrally with the crank case and is provided with the second bearing in the tubular portion, the drive shaft penetrating through the tubular portion, a center axis of the crank shaft is positioned between a vicinity of one end of the tubular portion and a vicinity of the other end of the tubular portion in an axial direction of the drive shaft, and a center axis of the drive shaft is positioned between a vicinity of one end of the first bearing and a vicinity of the other end of the first bearing in an axial direction of the crank shaft. That is, the center axis of the crank shaft may be positioned in the vicinity of one end of the tubular portion, in the vicinity of the other end of the tubular portion, or between the opposite ends of the tubular portion in the axial direction of the drive shaft. Also, the center axis of the drive shaft may be positioned in the vicinity of one end of the first bearing, in the vicinity of the other end of the first bearing, or between the opposite ends of the first bearing in the axial direction of the crank shaft.
According to a structure described above, the drive shaft can be placed close to the crank shaft so that a crank pin, a crank web, and a connecting rod do not come into contact with the drive shaft when they are rotating. Consequently, the engine can be made small. In addition, the rigidity of the tubular portion can lessen relative displacement between the drive shaft from which the rotational driving force is output and the crank shaft at which the rotational driving force is generated. Accordingly, the drive shaft and the crank shaft can be firmly supported.
In the power transmission device of an engine, a mounting portion may be formed integrally with the crank case for securing the crank case to a vehicle body frame of a vehicle, and the tubular portion may be positioned between the crank shaft and the mounting portion. According to a structure described above, strength of the portion supporting the crank shaft or strength of the mounting portion is ensured by the rigidity of the tubular portion. Accordingly, the crank shaft, and the drive shaft can be firmly supported by the vehicle body frame.
It is preferable that in the power transmission device of an engine, a part of a fitting portion for fitting the first bearing forms a side wall portion of a crank chamber, and the side wall portion, a peripheral wall portion surrounding an outer periphery of the crank shaft, and the mounting portion are integrally connected through the tubular portion.